Translations:Geometric seismology/7/en

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In many rocks, especially the crystalline types, the velocity at any point is dependent on the direction of propagation of the wave at that point. For instance, the vertical velocity of propagation can differ from the horizontal velocity of propagation at a given locality. When this is the case, the rock is called anisotropic. Unless otherwise stated, we will deal only with rocks that are isotropic - that is, those for which the propagation velocity of seismic waves is independent of direction. A good example of an anisotropic rock is Iceland spar, which is basically a clear cleaved fragment of a completely colorless (icelike) form of calcite. Iceland spar displays the classic cleavage form of calcite, the rhombohedron, and it best demonstrates the property of calcite known as double refraction.